This invention relates to the method of producing an aluminum nitride article and, more particularly, relates to a method of producing an aluminum nitride article utilizing a platinum catalyst to enhance the removal of carbon.
Aluminum nitride has been of interest recently for electronic packaging applications because of its high thermal conductivity, thermal expansion matching with silicon, low dielectric constant (8.5) and high electrical resistivity.
The present invention is particularly suitable for co-fired electronic packages, also known as substrates, but has applicability to aluminum nitride articles in general. The remaining discussion will focus on co-fired aluminum nitride electronic packagages but it should be understood that the present invention has applicability to other aluminum nitride articles.
In one co-firing process, the aluminum nitride is formed into greensheets (comprised of aluminum nitride particles in an organic binder), vias are punched, metallization paste (comprised of metallic particles in an organic binder ) is screened or extruded onto the greensheets and into the vias, the greensheets are stacked and laminated to form a substrate in the green state, and then the green substrate is sintered to densify the aluminum nitride layers and the metallization. "Co-fired" means that the metallic paste is sintered during the same sintering schedule as the aluminum nitride body. The metallization for aluminum nitride substrates is typically tungsten but may also be molybdenum or a mixture of tungsten and molybdenum. In addition, instead of forming the aluminum nitride body by using greensheets, dry pressing may be used to form the aluminum nitride body.
Binder removal from aluminum nitride laminates during sintering is difficult due to the required use of highly reducing (e.g., forming gas) ambients. When retained carbon levels are high at elevated temperatures (e.g., greater than 1200 degrees C.), the refractory metallurgy will carburize to form a carbide. For example, if tungsten metallurgy is used, the tungsten forms tungsten carbide (WC). The carbide, WC in this case, can have a severe impact on electrical performance of the metallurgy.
The carbon removal process is further complicated in the aluminum nitride system in that both aluminum nitride and sintering additives used can have substantial equilibrium vapor pressure at the sintering temperature. To attain close to 100% densification, the aluminum nitride laminates are enclosed in either a refractory metal box or a box made from unsintered aluminum nitride laminates which, for convenience, could be aluminum nitride kerf. The enclosed configuration creates additional problems in binder removal since the exchange of both reactants and by-product of binder removal are greatly reduced, thereby resulting in substantially higher retained carbon levels.
To accommodate the dual needs of binder removal and conservation of aluminum nitride and the sintering additives, a two-step firing cycle is utilized. First, the aluminum nitride laminates are subjected to binder burnoff by placing them in a furnace with a forming gas ambient at a temperature of about 900-1400 degrees C. After holding at this temperature for 2 to 6 hours, the aluminum nitride laminates are cooled to room temperature. The temperature for binder burnoff is insufficient to adversely affect densification during sintering. The second step is to sinter the aluminum nitride laminates by placing them in a box or container in a furnace with a forming gas ambient at a temperature of about 1550 to 1700 degrees C. After holding at this temperature for 5 to 8 hours, the aluminum nitride laminates are cooled to room temperature.
Such a two-step process is effective to remove carbonaceous residues while preventing the volatile components from evaporating. But, the two-step process also increases the cost of producing aluminum nitride products and reduces throughput.
Accordingly, it would be desirable to have a single-step process for producing aluminum nitride laminates.
Dolhert U.S. Pat. No. 5,256,609, the disclosure of which is incorporated by reference herein, discloses that for ceramics processed in reducing atmospheres, effective binder removal (and therefore also removal of residual carbon) can only be accomplished by use of a catalyst, a prolonged low temperature heating schedule, a wet gas atmosphere or a special binder. Dolhert does not disclose any particular catalyst to assist in effective binder removal. The prolonged low temperature heating schedule is not economically feasible while the wet gas atmosphere is not possible with the aluminum nitride materials contemplated by the present invention. Dolhert's invention relates to the provision of an atactic polypropylene-containing binder material.
Chance et al. U.S. Pat. No. 5,147,484, the disclosure of which is incorporated by reference herein, discloses the formation of mulitlayer ceramic substrates having copper metallurgy which are sintered in an air ambient. The ceramic material comprises a crystallizable glass. Zinc, platinum or chromium are added to the copper. The binder is decomposed and eliminated while the zinc, platinum or chromium combines or alloys with the copper. The platinum prevents oxidation of the copper-based conductor surface during binder burnout while the zinc or chromium forms a self-limiting surface oxide.
Kasori et al. U.S. Pat. No. 4,883,780, the disclosure of which is incorporated by reference herein, discloses an aluminum nitride body made from a composition comprising a binder, aluminum nitride powder, a sintering aid and at least one transition element (or a compound containing the transition element) from Groups IVa, Va, VIa, VIIa and VIIIa. Among the transition elements in these Groups are Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt. The composition, after being formed into an article, was placed in a carbon container and sintered in a nitrogen ambient. Kasori suggests the addition of the above transition elements to achieve improved densification and thermal performance and, additionally, for coloration of the sintered body.
Various solutions have been proposed for adding noble metals and other metals to enhance the removal of carbon from ceramic substrates.
Cowan, Jr. et al. U.S. Pat. No. 4,778,549, the disclosure of which is incorporated by reference herein, discloses the addition of a noble catalyst (Ru, Rh, Pd, Os, Ir, or Pt) to greensheets comprising a binder material and glass, glass-ceramic or ceramic particles. Binder burnout is accomplished in a wet nitrogen ambient. The noble catalyst catalyzes the removal of the binder resdiues.
Schwarz et al. U.S. Pat. No. 4,810,463, the disclosure of which is incorporated by reference herein, discloses the addition of nickel or tungsten, in the form of nickel salts and tungsten oxide, to impregnate a mass of alumina particles. The nickel or tungsten act to catalyze the removal of carbonaceous binder residues during binder burnoff in hydrogen or hydrogen/steam ambients.
Brownlow et al. U.S. Pat. No. 4,474,731, the disclosure of which is incorporated by reference herein, discloses the addition of nickel or palladium compounds to ceramic materials such as silica based materials. The nickel or palladium compound is mixed in with the polymeric binder before mixing with the ceramic materials. During the burnoff of the binder material, a wet hydrogen atmosphere is used. The nickel or palladium serves to catalyze the removal of carbonaceous binder residues during the binder burnoff.
Notwithstanding the above efforts, there still remains a need to improve the sintering of aluminum nitride bodies so that carbon residues remaining from binder removal during binder burnoff can be effectively removed.
Accordingly, it is a purpose of the present invention to have an improved method for the effective removal of carbonaceous residues remaining from binder removal.
It is a further purpose of the present invention to have a single-step method for the effective removal of carbonaceous residues and the effective sintering of aluminum nitride articles.
These and other purposes of the invention will become more apparent after referring to the following description of the invention.